Systematic use of solar energy can yield a large saving in the cost of winter space heating. If the house has good insulation, you may model it as losing energy by heat steadily at the rate 3000 W on a day in April when the average exterior temperature is 5°C, and when the conventional heating system is not used at all. The passive solar energy collector can consist simply of very large windows in a room facing south. Sunlight shining in during the daytime is absorbed by the floor, interior walls, and objects in the room, raising their temperature to 30°C. As the sun goes down, insulating draperies or shutters are closed over the windows. During the period between 5:00 P.M. and 7:00 A.M. the temperature of the house will drop, and a sufficiently large “thermal mass” is required to keep it from dropping too far. The thermal mass can be a large quantity of stone (with specific heat 700 J/kg.°C) in the floor and the interior walls exposed to sunlight. What mass of stone is required if the temperature is not to drop below 22°C overnight?
A 750 W heating coil designed to operate from 220 V is made of a copper wire 0.500 mm in diameter.
Assuming that the resistivity of the wire remains constant at room temperature, find the length of wire used.
A current in a coil having air in the interior creates a magnetic field = 0 . Describe qualitatively what happens to the magnitude of when (a) paramagnetic materials and (b) ferromagnetic material are placed in the interior of the coil.
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